Tunable broadband mode converters play an important role in wavelength division multiplexed (WDM) optical communication systems. They may be used, for example to dynamically convert a lightwave signal propagating in one mode of a “few mode” fiber into another spatial mode. Such coupling is attractive to alter the path the lightwave signal takes, since the alternate path (defined by the other spatial mode in the fiber) may have preferred dispersion, nonlinearity or amplification properties. An example of this is a higher-order-mode (HOM) dispersion compensator, where light in an entire communication band is switched from an incoming LP01 mode to a higher-order mode such as the LP11 or LP02 mode.
In a general sense, long-period gratings (LPGs) are mode-conversion devices that provide phase-matched coupling to transfer power from one mode of an optical fiber to another. This has proven to be especially useful for coupling between a guided mode and a cladding mode of ordinary transmission fibers so as to create a wavelength-selective loss. In optical communication systems, LPGs have been used extensively for realizing devices that offer wavelength-selective attenuation of a WDM communication signal. Dynamic tuning of the spectral characteristics of LPGs has been proposed and a variety of dynamic tuning techniques have been demonstrated. In particular, LPGs that couple the core mode to a cladding mode can be dynamically tuned by modulating the refractive index of an outer or inner cladding material that interacts with the cladding mode of the fiber. A microbend-induced fiber grating (MIG) is one type of dynamic LPG where the perturbation in refractive index is induced by periodic “microbending” of the fiber. In particular, the coupling strength of a MIG is tunable by changing the strength of a propagating acoustic wave or by changing the pressure applied to the fiber (e.g., pressing the fiber with a corrugated surface of a defined periodicity).
As will be described in detail below, MIGs have several advantageous device applications. For example, when one of the co-propagating modes is the fundamental mode of a single mode fiber and the other is a cladding-guided mode, MIGs yield wavelength-dependent loss spectra when broadband light is transmitted through the single mode fiber. Wavelength-dependent loss is known to be useful for several device effects, such as gain equalization filters, spectral shapers for broadband lightwave devices, amplified spontaneous emission filters, loss filters for stabilizing the operating wavelength of fiber lasers, etc. On the other hand, if both of the co-propagating modes are guided in the core region of a fiber, MIGs can be used to realize efficient mode conversion, as noted above, which has applications such as higher-order mode conversion, variable optical attenuation, etc.
One drawback to the use of MIGs is their inherent polarization sensitivity, even when the grating is induced in a perfectly circular fiber. The mode depictions in FIGS. 1 and 2 can be used to explain this phenomenon. A microbend-induced fiber grating, as noted above, will couple a circularly symmetric and polarization degenerate mode (such as the LP01 mode shown in FIGS. 1(a) and (b)) with anti-symmetric LP1m modes (such as the LP11 mode of FIGS. 1(a) and (b)), where m defines the radial order of the anti-symmetric mode. Referring to FIG. 2, the LP11 mode is shown as possessing a four-fold degeneracy including the vector modes TE01, TM01 and the odd and even HE21 modes. In any fiber waveguide possessing radial index variations (which are necessary to define a core/cladding boundary), these four modes are known to exhibit slightly different propagation constants. Thus, coupling with a microbend fiber grating of a given grating period Λ results in exhibiting slightly different resonant wavelengths for each one of the different modes. Since different polarization orientations of the fundamental mode will result in different excitation levels for the four modes, the resulting coupling spectrum will also be polarization dependent—an unwanted result since it severely restricts the applicability of MIGs in fiber optic systems, where a “polarization insensitive” response is often necessary condition.
Prior attempts at reducing the polarization sensitivity of MIGs have generally fallen into three classes: (1) inducing microbends along two orthogonal transverse axes of the fiber, in one case by helically winding thin wires around a fiber to generate circularly symmetric microbends; (2) forming MIGs in extremely thin fibers (results in coupling only to the odd/even HE modes) and (3) introducing polarization diversity into the system, using external components to compensate for polarization-dependent losses. Looking at the first solution, it has been found to be limiting in the sense that it involves the precision machining of expensive and complex corrugated blocks with tight angular tolerances. Additionally, it is necessary to ensure that no polarization rotation occurs as the light traverses from one set of microbends to an orthogonal set of microbends. The use of helical microbends, as also proposed, requires individual assembly for each device (with each device requiring a high level of precision) and cannot provide “strength tuning”—the fundamentally attractive feature of MIGs. The use of extremely thin fibers, as required in the second class of solutions, is not practical for “real world” system applications and can only be used with acousto-optic configurations since the action of pressing a corrugated block against an extremely thin fiber introduces a host of reliability and yield issues. The third class of solutions (polarization diversity) requires the use of a device such as a Faraday rotator mirror to rotate the state of polarization (SOP), in association with a circulator and polarization beam splitter to form a pair of orthogonal signals. Indeed, a pair of essentially identical MIGs would be required, each acting on a separate one of the orthogonal components. This scheme is considered to add substantial loss, as well as cost and size, to the system.
Thus, a need remains in the prior art for a microbend-induced fiber grating that is polarization insensitive and useful in a variety of system applications, providing polarization insensitivity regardless of the configuration used to induce the microbends in the fiber (e.g., acousto-optical fiber, corrugated blocks, permanently etched gratings, etc.).